1. Field of the Invention
This invention relates to a process of determining and/or automatically controlling the degree of orientation of tubular films made in film-blowing plants comprising an extruder having a film-blowing head and means for internally cooling, flattening and taking off the tubular film as well as means for controlling the rate of exchange of the air for internal cooling, the speed of the extruder, the temperature of the extruded plastic being extruded and/or the take-off velocity.
2. Description of the Prior Art
Between the annular die of the film-blowing head and the so-called frost line, which is usually closely spaced below the calibrating basket that confines the inflated film bubble, the extruded molten tubular film is inflated to form a film bubble, which above the frost line has the diameter of the tubular film to be made. The film bubble is inflated by means of cooling air, which is supplied and withdrawn in lines extending through the film-blowing head. The rates at which the cooling air is supplied and withdrawn, i.e., the rate at which said air is exchanged, and the cooling rate as well as the pressure in the bubble can be controlled. The plastic tubing extruded through the film-blowing head initially preserves its as-extruded diameter and is subsequently inflated in the shape of a tapered champagne glass to form the film bubble having the desired final diameter. The stretching of the film bubble is virtually terminated at the frost line, at which the resulting elongation is virtually "frozen". Particularly in its flaring portion having the shape of a tapered champagne glass the film is stretched in its peripheral or transverse direction and in its longitudinal direction so that the plastic molecules are correspondingly oriented. Whereas the inflation of the extruded tubular film to form the film bubble results in a transverse and longitudinal stretching of the tubular film, an additional longitudinal stretching can be effected by a suitable control of the take-off velocity.
The quality and the strength properties of the tubular film which is made will essentially depend on the longitudinal and transverse stretching effected during the cooling phase, i.e., in the region in which the tubular film flares as it is inflated. That result will depend not only on the extent of the longitudinal and transverse stretching but particularly also on the ratio of the velocities of the stretching in the longitudinal and transverse directions; that ratio is described as the degree of orientation.
The ratio of the stretching in the longitudinal and transverse directions, the degree of orientation and the diameter of the tubular film which is made will be influenced by the position of the so-called frost line. The elevation of the frost line may be influenced by various processing parameters, such as the temperature of the plastic composition, the speed of the extruder, the rate of exchange of the cooling air, the temperature of the cooling air, and the take-off velocity. The position of the frost line may even change as a result of a change of the room temperature. On principle, the frost line should be closely spaced below the calibrating basket.
It is known to automatically control the diameter and the stretch ratio of the tubular film by a control or automatic control of the elevation of the frost line relative to the film-blowing head and/or to the calibrating basket.
In most film-blowing plants the position of the frost line is only visually monitored by the operators. In dependence on the experience of the operators the elevation of the calibrating basket, the rates at which the air for internal cooling is supplied and withdrawn and/or the production rate of the extruder are adjusted in dependence on the detected elevation of the frost line. But such adjustments may involve considerable errors because they depend on the skill and experience of the operators.
German Patent Specifications 27 21 609 and 28 31 212 disclose the automatic control of film-blowing plants by methods in which particularly for maintaining a constant predetermined width of the film the position of the frost line is measured and final control elements for influencing the thickness of the film and/or the diameter of the tubular film are controlled in dependence on the deviations which have been detected. But said known methods involve a problem relating to the devices and sensors for detecting the frost line.
It is known to detect the frost line by means of temperature sensors for detecting the temperature of the film by a measurement of the infrared radiation in regions below and above the frost line. But such sensors are highly susceptible to being soiled and this may give rise to trouble in the operation. Finally, temperature sensors cannot be used to detect the temperature of very thin transparent films.
In other known methods the position of the frost line is computed from the energy balance in consideration of the feed rate of the raw material, the production rate, the width and thickness of the film, the temperatures of the cooling air and other controlling parameters. But such methods are not accurate because they depend on properties of the raw materials and the corresponding data are often unknown and can be estimated only with difficulty. Besides, it is difficult to take the influence of the film coefficient of heat transfer between the air and the film into account. For this reason a computation which is sufficiently accurate for practical use is not possible.